Turbine engines, such as those used as aircraft turbojets or turbofans typically comprise from front to rear, in the direction of fluid flow in the engine, fan, compressor and turbine sections within an engine casing. These engine sections comprise a rotating fan, compressor, and turbine that are axially mounted on one or more coaxial shafts for rotation about a central axis of the engine. The shafts are supported by at least two bearing assemblies. These bearing assemblies permit rotation of the shafts, and in turn the fan, compressor and turbine. Typically, the front-most bearing assembly in the direction of fluid flow in the engine also prevents axial movement of the shaft within the case and is referred to as a "thrust bearing assembly".
While thrust bearing assemblies are typically machined to tight tolerances, small amounts of play between the ball bearings and the bearing races within the housings exist. This play is undesirable as it causes unnecessary noise and vibration of the engine, when the engine is in operation. It is particularly noticeable at low engine speeds.
Much of this play can be eliminated by exerting a load in an axial direction either forward or rearward on the bearing. Such a load say, for example in the forward direction, may be transferred to the bearing through the shafts. The forward load may, for example be exerted by pressurized gases from the compressor exerting a forward force on the rear portion of the compressor section, which in turn exerts a forward force on the shafts. However, due to size constraints on the engine and performance requirements of the compressor section, the amount of pressure exerted in conventional engine designs may not be enough to place sufficient forward load on the thrust bearing in order to sufficiently reduce the play in the bearing assembly and accompanying noise and vibration. Moreover, the force exerted by such pressurized gases will vary depending on the rotational speed, pressure and temperature within the engine.
One solution to address this problem has been to include a rotating balance piston mounted on the shaft, aft the compressor section of the engine, as will be described in greater detail below. This balance piston is located between a high and low pressure region within the engine. The pressure differential on the piston exerts a forward force on the shaft, and in turn on the thrust bearing. The pressure differential across the piston, however, also varies with the rotational speed and temperature of the engine. The higher the speed and temperature of the engine the greater the pressure differential across the piston.